It's a vector field! It has 2 dimensional inputs and 2 dimensional outputs, so it doesn't fit on your traditional graph.

    f(x,y) = (c * x, c * y)
    f(x,y) = c * (x,y)
    f(P) = c * P

If you give some thought to what `c` is doing to each point of your plane (start with the origin!), I bet that graph might make a bit more sense. :)